webrtc RTP包接受与重传

RTP包处理

void WebRtcTransportImp::onRtp(const char *buf, size_t len, uint64_t stamp_ms) {
    _bytes_usage += len;
    _alive_ticker.resetTime();

    RtpHeader *rtp = (RtpHeader *)buf;
    // 根据接收到的rtp的pt信息，找到该流的信息
    auto it = _pt_to_track.find(rtp->pt);
    if (it == _pt_to_track.end()) {
        WarnL << "unknown rtp pt:" << (int)rtp->pt;
        return;
    }
    it->second->inputRtp(buf, len, stamp_ms, rtp);
}

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正常包

void WrappedRtpTrack::inputRtp(const char *buf, size_t len, uint64_t stamp_ms, RtpHeader *rtp) {
#if 0
    auto seq = ntohs(rtp->seq);
    if (track->media->type == TrackVideo && seq % 100 == 0) {
        //此处模拟接受丢包
        return;
    }
#endif

    auto ssrc = ntohl(rtp->ssrc);

    // 修改ext id至统一
    string rid;
    auto twcc_ext = track->rtp_ext_ctx->changeRtpExtId(rtp, true, &rid, RtpExtType::transport_cc);

    if (twcc_ext) {
        _twcc_ctx.onRtp(ssrc, twcc_ext.getTransportCCSeq(), stamp_ms);
    }

    auto &ref = track->rtp_channel[rid];
    if (!ref) {
        _transport.createRtpChannel(rid, ssrc, *track);
    }

    // 解析并排序rtp
    ref->inputRtp(track->media->type, track->plan_rtp->sample_rate, (uint8_t *)buf, len, false);
}

重传包

void WrappedRtxTrack::inputRtp(const char *buf, size_t len, uint64_t stamp_ms, RtpHeader *rtp) {
    // 修改ext id至统一
    string rid;
    track->rtp_ext_ctx->changeRtpExtId(rtp, true, &rid, RtpExtType::transport_cc);

    auto &ref = track->rtp_channel[rid];
    if (!ref) {
        // 再接收到对应的rtp前，丢弃rtx包
        WarnL << "unknown rtx rtp, rid:" << rid << ", ssrc:" << ntohl(rtp->ssrc) << ", codec:" << track->plan_rtp->codec
              << ", seq:" << ntohs(rtp->seq);
        return;
    }

    // 这里是rtx重传包
    //  https://datatracker.ietf.org/doc/html/rfc4588#section-4
    auto payload = rtp->getPayloadData();
    auto size = rtp->getPayloadSize(len);
    if (size < 2) {
        return;
    }

    // 前两个字节是原始的rtp的seq
    auto origin_seq = payload[0] << 8 | payload[1];
    // rtx 转换为 rtp
    rtp->pt = track->plan_rtp->pt;
    rtp->seq = htons(origin_seq);
    rtp->ssrc = htonl(ref->getSSRC());

    memmove((uint8_t *)buf + 2, buf, payload - (uint8_t *)buf);
    buf += 2;
    len -= 2;
    ref->inputRtp(track->media->type, track->plan_rtp->sample_rate, (uint8_t *)buf, len, true);
}

处理包

    RtpPacket::Ptr inputRtp(TrackType type, int sample_rate, uint8_t *ptr, size_t len, bool is_rtx) {
        auto rtp = RtpTrack::inputRtp(type, sample_rate, ptr, len);
        if (!rtp) {
            return rtp;
        }
        auto seq = rtp->getSeq();
        _nack_ctx.received(seq, is_rtx);
        if (!is_rtx) {
            // 统计rtp接受情况，便于生成nack rtcp包
            _rtcp_context.onRtp(seq, rtp->getStamp(), rtp->ntp_stamp, sample_rate, len);
        }
        return rtp;
    }